Today on Galileo - June 24, 1997
The number of observation during the third day of this encounter period picks up a little bit, but continues to be light compared to the days to come. The UVS (Ultraviolet Spectrometer) instrument completes its observation of the Ganymede neutral torus. After three days of neutral torus observations, you have to be asking yourself, what are all these neutral torus observations about? For the answer, we asked one of the UVS experts....
It turns out that these observations are looking for oxygen and hydrogen (and possibly other elements) that are released when solar photons (i.e. light) strike the surface of a particular satellite. The oxygen and hydrogen produce a tenuous atmosphere around the satellite and form the neutral torus in the satellite's orbit. Measuring the distribution of these particles tells us something about the rate at which material is being lost from the surface of the satellites. Major changes from orbit to orbit in these torus measurements could be caused by something changing on the surface of the satellite. If Galileo can see any dramatic changes in a satellite's neutral torus, that could be a clue that the satellite is geologically active!
Late in the day, the UVS turns its attention back to Ganymede, but this time looks at the satellite itself at solar phase angles not viewable from Earth (i.e. with the Sun illuminating Ganymede from an angle that cannot be seen from Earth). Finally, the PPR (Photopolarimeter Radiometer) performs a regional observation of Jupiter's atmosphere.